Carnival for several years had the Paradise, a totally non-smoking ship. Eventually they dropped that policy. It was never confirmed, but rumors on the boards said that they learned that non-smokers are also usually light drinkers and light gamblers, and their revenues from the bars and casino dropped off significantly.
Cruise ships are highly dependent on those two areas. Keep an eye on Princess' "great experiment" and see what happens. Or better yet, if you are a non-smoker, be sure to hit the bars and casinos heavily to preserve the non-smoking policy!